Tetraselmis

Ultrastructure

The four flagella are each coated with three discrete layers of organic
(polysaccharide-containing) scales; pentagonal scales, rod scales and hair scales. The theca is
also composed of polysaccharide scales; because the individual scales are fused together, the
scale-based structure of the theca is not apparent in all preparations. All scales are formed in the
Golgi apparatus, which is found in the anterior end of the cell adjacent to the flagellar bases.

The flagellar apparatus consists of four flagellar bases, four microtubular roots, and two massive
striated roots (rhizoplasts), located at the base of the apical depression. The arrangement of these
elements approaches 180° rotational symmetry (a kind of radial symmetry). The flagellar bases
are nearly parallel to each other, and their proximal ends are mounted in nearly a straight line.
Complex structures, sometimes called "half-desmosomes", link each of the microtubular roots to
the cell membrane a short distance from the root origin. The rhizoplasts, which connect the basal
bodies to the nucleus, are contractile, in response to changes in calcium-ion concentration.
Centrin is a, perhaps the, major structural protein in rhizoplasts.

Few differences are noted between species in scale and flagellar apparatus features.

The pyrenoid is usually located immediately posterior to the nucleus. The pyrenoid typically is
surrounded by starch grains, and is penetrated by cytoplasmic channels which may have lobes
of the nucleus within. The structure and content of these channels supply the principal characters
used to identify subgenera of Tetraselmis.

Cell division is marked by a "metacentric" spindle. The spindle poles form in the vicinity of the
rhizoplasts, which disassemble. Flagella are shed, but the flagellar bases persist and, after
replication and migration, lie lateral to the spindle poles. At cytokinesis, microtubules arising
from the flagellar base complexes ("phycoplast" microtubules) line the cleavage furrow that
eventually separates the cells.